Circuit arrangement including two-valley semiconductor device

ABSTRACT

A two-valley semiconductor device, biased below the threshold voltage for triggering or sustaining oscillations, is triggered by input pulses to release an output that is an amplified or regenerated form of the input. In another embodiment an analogue signal input is encoded into a pulse position modulation train.

United'Sta tes Patent Appl. No. Filed Patented Assignee 542,170, Apr. 12, 1966,51pwgbandoned.

CIRCUIT ARRANGEMENT INCLUDING TWO- VALLEY SEMICONDUCTOR DEVICE OTHER REFERENCES An article titled Some Aspects of Gunn EfiectOscillators written by P. N. Robson & S. M. Mahrous in the December 1965 issue of The Radio & Electronci Engineer, pp

345- 351. A copy of this article is located in 331/107 (G) in Group 250.

Primary ExaminerStanley T. Krawczewicz 2 Claims, 10 Drawing Figs Attorneys-R. .l. Guenther and Arthur J. Torsiglieri US. Cl 307/260,

307/252, 307/258, 307/283, 307/286, 307/322, ABSTRACT: A two-valley semiconductor device, biased 328/164, 331/107, 331/1 15 below the threshold voltage for triggering or sustaining oscilla- Int. Cl. H03k 5/01 tions, is triggered by input pulses to release an output that is an Field 01 Search 307/206, amplified or regenerated form of the input. In another cm- 252, 258, 260, 283, 286, 322; 328/164, 13; bodiment an analogue signal input is encoded into a pulse 331/15, 107(G) position modulation train.

DC VOLTAGE SOURCE H l2 [l0 PULSE WO ALLEY TRAIN SEMICONDUCTOR SOURCE DEVICE LOAD A PATENTEU JANZS ISTI SHEET 1 [IF 2 FIG.

DC VOLTAGE SQURCE \H |2 [I0 PULSE TWO VALLEY TRAIN SEMICONDUCTOR SOURCE DEVIYCE LOAD FIG. 2A

FIG. 25 I IN FIG. 2C

TIME

TIME

TIME

FIG. 4

DC I32 VOLTAGE SOURCE 3| TWO VALLEY SEMICONDUCTOR DEVICE LOAD lNl/ENTOR M1 UENOHARA BY ATTORNEY PATENTEUJANZBIBII V 3.658.923

' sum 2 (IF 2 F16. 5c YL This application is-a continuation-in-part of my copending application, Ser. No. 542,170, filed Apr. 12, 1966, now aban? doned, and assigned-to Bell Telephone Laboratories, Incorporated. t

BACKGROUND OF THE INVENTION This invention relates to' circuit arrangements employing services of a compound semiconductor in which instabilities are produced by the transfer of hot electrons between two conduction-band valleys with greatly difference mobilities and separated in energy. Such'devices are typically described as two-valley semiconductor devices and the theory of such devices is described in detail in a series of papers in the Jan.

I966 issue of the IEEE Transactions on Electron Devices, Volume ED-l 3, Number I. p

More particularly, the invention relates to pulse generation arrangements employing two-valley semiconductor devices. As is set forth in thepapers mentioned above, when the voltage applied to opposite ends of a suitable sample of an appropriate two-valley semiconductor, such as gallium arsenide, is increased, the average sample current increases almost linearly to a maximum value and then drops sharply ,to a fraction of this maximum value. Above this criticalvoltage the average sample current remains essentially constant. If the instantaneous current waveform is analyzed, it is found to be oscillating periodically at a frequency related to the sample a length. This critical, voltage rnarkingtheldrop in sample current and the initiation of oscillations ,will be termed the threshold voltageof oscillations V,.

It is now believed that the oscillations are associated with the nucleation adjacentto the negative'electrode and travel approximately equal to the sample length lbetween opposite contacts divided by the carrier drift'velocity v SUMMARY OF THE INVENTION v DETAILED DESCRIPTION With reference now to the drawing, FIG. 1 shows in block form the basic elements of a pulse regenerator in accordance with the inventionhThe two-valley semiconductor device 10, which typically would be a gallium arsenide wafer provided with a pair of spaced electrodes, is connected serially with a DC voltage source 11, a source 12 of the signal pulse train to be regenerated,- and a load orv utilization branch 13. The voltage bias W provided by voltage source 11 is arranged to be sufficiently less than the threshold voltage for oscillations V that the likelihood that oscillations will be triggered by noise rather than a true signal pulse of the pulse train is sufficiently also be less than the voltage V needed to sustain oscillations once'initiated. .As is discussed in my copending application,

Ser. No. 542,168 filed Apr. 12, 1966, U5. Pat. No. 3,508,210, contemporaneously with the parent application and assigned to the same .assignee as the instant application, I have found that, in these devices, oscillations once initiated tend to be sustained so long as. there is maintained a bias voltage typically I equal to about 95 percent of thethreshold value V Since in 'the presentinstance, it is desirable that the oscillations be terminated as soon as possible after termination of a real pulse of the pulsetrain, thebias voltage should be close to but less than the sustainingvoltageV typically about 94 percent of the In accordance with a preferred embodiment of the present invention, a two-valley semiconductor device is included in a circuit arrangement which includes a DC voltage source which establishes a steady bias on the device less than the threshold voltage V a signal source supplying a pulse train which has become distorted intransmission and which it is desired to regenerate and amplifyfree of such distortion, and a load or utilization branch in which is developed the desired regenerated and amplified pulse train. The two-valley device may be biased above or below the sustaining voltage greater than V,,. However, by biasing the device at a voltage greater than V but less than V an incoming pulse having a duration less than .the transit time 1- can be made to triggerthe device to generate an output pulse of prescribed duration'r. I

In a related embodiment of the invention, a two-valley seniiconductoris included in a circuit with a suitable voltage bias source, asourceof an analogue signal to be encoded into a pulse position modulation train, and a load or utilization branch.

It is characteristic of these embodiments that they are basically very simple toinstrument, have very high speeds, can handle appreciable powers, and can produce amplification of the desired pulse train. a

DRAWINGQDESCRIPTION threshold voltage..The bias voltage V may be above or below the sustaining voltageof the domain V If V,, is above V,,, a high-field domain,-once formed,*willpersist until it reaches the positive; contact even if the superposed pulse voltage falls below V In some instances,'where the amplitude of signal I which had been supplied the transmitted signal after reception. In other instances, itmay be the antenna used to pick up :theradiated signal pulse tiain.

The nature of utilization apparatusl3 also would depend on the use which the pulse regeneration is to serve. As will appear below, theoutput informationcan be derived-in a variety of forms. Moreovenit'should be apparent that the desired output can be derived by connecting the load across the semiconductive element. In FIG. 2A-is depicted thewaveforms of the various applied voltages of interest. The dotted line 21 depicts the voltage V,; supplied by source 11 on which is impressed the varying voltage supplied by the pulse train source such that the eifective voltageapplied. has the waveform shown by the solid line 22. This varyingvoltage corresponds to apulse train originally having .the waveform shown in FIG. 2B, which has been distorted in transmission. The threshold voltage for initiating oscillations ,V is shown by the dashed line 23.

In FIG. 2Bthere is plotted on the same timescale as is used in FIG. 2A the average current flowing through the sample. It

can be noted that the average current falls sharply as the applied voltage, increases beyond the threshold voltage and is thereafter relatively insensitive to variations in the applied voltage until the applied voltage drops below the threshold voltage. Actually the oscillations and the low average current condition persist until the applied voltage falls below the value needed to sustain oscillations, which as previously described is slightly less than the threshold voltage. This phenomenon results because of transient effects, but the lower the product nL (where n is charge carrier density in the sample and L is the length of the sample between electrodes) the more nearly the sustaining voltage of oscillations equals the threshold voltage. Accordingly, the difference can be kept small by appropriate design of the sample and the associated circuitry to minimize excess reactance. Conversely, the difference can be increased if desired by increasing nL.

It is characteristic that during the interval when the average current is low there will also be oscillatory current flow in the circuit superposed on a steady current. In FIG. 2C, there is depicted the envelope of the oscillatory current I that flows in I the circuit.

It is obvious that there are alternative possibilities for deriving the output information from the original pulse train.

First, since the average current flowing through the circuit has the waveform of the original pulse train, (except for the inversion in sign) the average voltage across a resistive load will also have the desired waveform and accordingly such voltage can be utilized for further processing as the signal pulse train. In such a case, it would be advantageous to filter out the oscillatory component superposed on the steady component.

Additionally, since the envelope of the oscillatory current flowing in the circuit corresponds to the desired regenerated pulse train, the envelope of the oscillatory voltage across a resistive load will have the desired output waveform and, accordingly, such voltage can be utilized for further processing as the signal. In this instance, it will be advantageous to remove the steady component.

Which of the two techniques described for deriving the desired regenerated pulse train is to be preferred depends on the nature of the further processing intended for the derived pulse train. When the oscillatory current is utilized, the derived pulse train comprises a train of bursts of oscillatory energy with the frequency of oscillations being governed by the geometry of the sample in the manner characteristic of two-valley semiconductor devices.

The foregoing discussion presupposes a duration of each output pulse, in response to an input pulse that extends above voltage V for substantially the same duration, which is greater than the domain transit time 1r, where where I is the sample length of the two-valley device between opposite contacts and 1/ is the domain drift velocity.

Because the domain transit time 1' is a fixed parameter, the circuit of FIG. 1 can be operated such to release pulses of duration r in response to incoming pulses of any varying duration less than 1'. Referring to FIG. 3A, assume that the two-valley device is biased at a voltage V above the voltage V for sustaining a domain. Then, whenever the waveform 24 exceeds the threshold voltage V a domain is formed in the twovalley device that persists during its transit time 1' even if in the interim the input waveform voltage drops below V During transit time 1-, the current in the load drops as shown in FIG. 33.

If the duration of all the triggering pulses superposed on the bias voltage are smaller than r, then all of the output pulses will have a uniform duration 1' regardless of nonuniformities of input pulses duration. This mode of operation has obvious advantages where the pulses of a pulse train to be regenerated each have durations on the order of 10-- to 10-- second, which are consistent with values of 1' that can be realized by presently known two-valley devices. Then the pulse width, as well as pulse height or amplitude can be regenerated.

In FIG. 4, there is shown schematically an arrangement whereby an analogue signal is encoded into a pulse position modulation train of pulses. The two-valley semiconductor device 31 of the kind previously described is connected serially with a DC voltage source 32, a source 33 of the analogue signal to be encoded, and a load or utilization branch 34. Again the voltage V of the source 32 is typically adjusted to maintain a bias across the element 31 slightly below the value at which oscillations initiated are sustained, and its inclusion permits obtaining amplification.

In FIGS. 5A through 5C are shown the waveforms of the relevant voltages and current in the circuit depicted. In FIG. 5A, the steady voltage V,, provided by voltage source 32 is shown by the dotted line 41 and the threshold voltage needed to be exceeded for the initiation of oscillations is shown by the broken line 42. The sustaining voltage of oscillations below which the applied voltage needs to fall to terminate the oscillations is not shown but advantageously is made close to the threshold voltage and is greater than the steady voltage V The solid line 43 depicts the effective applied voltage corresponding to superimposing the varying analogue voltage to be encoded on the steady bias.

In FIG. 5B is depicted the average current flowing in the circuit. In particular, it is characteristic that the average current drops sharply at a time corresponding to the time the applied voltage exceeds the threshold voltage.

In FIG. 5C is depicted the average across the load under the conditions depicted in FIG. 5A. Here too the leading edges 45 of the negative swings of the pulse train correspond to the times the applied voltage exceeds the threshold voltage.

As can be appreciated by workers in the pulse code an, this arrangement permits the encoding of the analogue signal into a pulse position code useful in specialized transmission systems.

It will be appreciated by a worker in the art that the specific circuit arrangements described have been simplified by elimination of all but the basic elements to facilitate exposition of my novel contribution. Typically, the operation will be at frequencies where strip lines, coaxial cables or wave guides will be advantageously used to interconnect the circuit elements with provision being made for shunting the high frequency currents across at least the DC voltage source. However, all of these techniques are well known to workers in the art and so have not been discussed in detail here.

Various embodiments and modifications other than those described herein may be made by those skilled in the art without departing from the spirit and scope of the invention.

I claim:

1. A circuit arrangement for encoding an analogue signal into a pulse position modulation code comprising:

a microwave frequency two-valley semiconductor oscillation. generator characterized by the capacity to form and to propagate successive electric field traveling domains in response to an applied DC voltage in excess of a threshold value;

means providing a bias voltage of magnitude less than the threshold voltage of oscillations of the device;

means for superposing on said bias voltage the analogue voltage to be encoded, the sum of the bias voltage and at least part of the analogue voltage being greater than the threshold value, whereby the semiconductive device is excited to an oscillatory state whenever the amplitude of the analogue voltage exceeds a prescribed level, thereby developing a pulse position modulation code composed of microwave frequency current having an envelope forming a series of pulses each having a duration determined by said analogue voltage; and means for deriving for utilization the pulse position modulation train developed. 2. A circuit arrangement for regenerating a pulse train from a source, which pulse train has become distorted in transmission, and to transmit to an appropriate load a pulse train relatively free of such distorting comprising: 

2. A circuit arrangement for regenerating a pulse train from a source, which pulse train has become distorted in transmission, and to transmit to an appropriate load a pulse train relatively free of such distorting comprising: a microwave frequency two-valley semiconductor oscillation generator characterized by the capacity to form and propagate successive electric field traveling domains in response to an applied DC voltage in excess of a threshold value, thereby to generate a microwave frequency output current; means for applying to the generator a bias voltage which is smaller than said threshold value; means for superposing on said bias voltage the pulse train to be regenerated; the sum of the amplitudes of the bias voltage and component voltage pulses of the pulse train being greater than the threshold voltage of oscillations of the generator; and means comprising said component voltage pulses for exciting the semiconductor oscillation generator to an oscillatory state, whereby the envelope of output microwave current from said generator is in the form of pulses each having a duration substantially equal to the duration of a corresponding voltage pulse of the pulse train. 